1. Field of the Invention
This invention relates to integrated circuit fabrication and, more particularly, to methods for forming metallization structures.
2. Description of the Related Art
The information described below is not admitted to be prior art by virtue of its inclusion in this Background section.
An integrated circuit includes numerous active and passive devices arranged upon and within a single substrate. In order to implement desired circuit functions, select devices or components of the integrated circuit must be interconnected. Metallization structures are often used to interconnect integrated circuit components. Metallization structures may be generally subdivided into two categories: laterally extending interconnect lines and vertically extending contacts or plugs. Interconnects are relatively thin lines of conductive material that largely extend parallel to the underlying devices. As the name implies, contacts are the metallization structures that actually contact the devices of the integrated circuit. Plugs mostly extend vertically between metallization levels.
Within each level of interconnect, metallization structures are separated from other structures on underlying or overlying levels, and from structures within the same metallization level, by dielectric materials. The dielectric materials prevent unwanted communication between separated metallization structures. In large part because of the difficulty in etching many metallization materials, metallization structures are often formed by first depositing the dielectric material which will separate the metallization structures and then patterning cavities in the dielectric material (i.e., metallization cavities) for the metallization structures. The metallization cavities patterned for interconnect structures are typically called trenches, and the metallization cavities patterned for plugs are typically called vias. Once the cavities are formed, metals can then be deposited in the cavities to form metallization structures. If necessary, the deposited metal can be planarized, a process that often involves chemical-mechanical polishing (CMP).
Despite the name, metallization structures are not required to actually be metals, but may instead be fabricated from any material sufficiently conductive to transmit an electrical signal (e.g., doped polysilicon, metal silicides, refractory metal nitrides). For metallization structures above the level of local interconnect, though, metals are the predominant metallization materials, and one of the most common metallization materials is aluminum. Aluminum is desirable as a metallization material because of, among other things, its relatively low resistance and good current-carrying density.
Aluminum is usually deposited using physical vapor deposition (PVD). PVD processing may also be known as sputter deposition, or sputtering. In general, sputter deposition may be considered to be any deposition process in which a material is deposited by sputtering the material from, e.g., a target composed of the material to be deposited. A typical method for sputtering a metal onto a substrate includes introducing an inert gas into a deposition chamber and forming a plasma that ionizes the inert gas by applying a potential between the substrate and the target. The ionized inert gas atoms are then attracted toward the target, and collide with the target with such force that atoms of the target are sputtered off. The sputtered atoms may then deposit on the substrate.
Sputtering can be used to deposit any variety of materials, including conductors, non-conductors, and high melting point compounds. Sputtering is advantageous because it may provide for good step coverage and accurate transfer of material composition from the target to the deposited metal. This last feature is particularly helpful when depositing alloys.
One process for forming a metallization structure incorporating aluminum involves first sputter depositing a titanium wetting layer into the cavity in which the metallization structure will be contained. The titanium wetting layer lines the sidewalls and base of the cavity. A bulk metal layer of aluminum is then sputter deposited onto the wetting layer to fill the cavity. The titanium wetting layer helps to minimize or avoid agglomeration of the aluminum bulk metal layer and provides for continuous metal coverage along the sidewalls and bottom of the cavity. The aluminum bulk metal layer serves as the primary conductive material of the resulting metallization structure.
Bulk metal layers composed of aluminum are typically deposited by either standard or collimated sputtering processes. Standard sputtering processes may be considered those sputtering processes that do not impart any significant degree of directionality to the sputtered atoms. Standard sputtering processes thus allow sputtered atoms to contact the deposition surface at a variety of angles, ranging from almost parallel through perpendicular. In contrast, collimated sputtering processes typically use a collimator arranged between the target and a deposition surface to block high impact angle atoms (e.g., those atoms having impact angles further from perpendicular) while allowing lower impact angle atoms (e.g., those atoms having impact angles closer to perpendicular) to pass through. Collimated sputtering processes, however, generally do not impart significant directionality to the sputtered atoms that are allowed to pass through the collimator.
Unfortunately, as the widths of metallization cavities continue to decrease and the aspect ratios of such cavities continue to increase, forming an adequate bulk metal layer becomes more difficult. One reason for this difficulty results from the buildup of deposited metal on the upper sidewalls of a metallization cavity. Because the majority of metal atoms deposited in a standard or even a collimated sputtering process will have impact angles deflected from perpendicular, the dielectric layer upper surface adjacent the cavity and on the upper sidewalls of the cavity may receive significantly more deposited metal atoms than the lower sidewalls and bottom of the cavity. This buildup on the dielectric layer upper surface adjacent the cavity and upper sidewalls of the cavity may result in metal overhanging, or shadowing, the lower cavity sidewalls and cavity floor. When this happens, deposited metal cannot reach the shadowed sidewall portions, and thus these areas may not receive sufficient metal coverage. This is particularly a problem for the lower portions of the cavity sidewalls, which are perhaps the portions most likely to be shadowed.
Furthermore, because the overhanging metal prevents deposited metal from reaching the shadowed portions of the cavity, metal may build up on the overhanging areas during deposition so much that the opening near the upper region of the cavity becomes closed. If the cavity was not filled before the opening was closed, a void (commonly known as a xe2x80x9ckeyhole voidxe2x80x9d because of its shape) can exist within the cavity. The presence of a void within a final metallization structure, of course, can be extremely detrimental to the performance of the structure.
In an attempt to resolve this problem, many processes have implemented hot sputter depositing of the bulk metal layer. Generally speaking, a metal can be either cold sputter deposited or hot sputter deposited. Cold sputter deposition processes deposit a metal at a temperature such that the deposited metal, upon deposition, cannot significantly reflow, and hot sputter deposition processes deposit a metal at a temperature such the deposited metal, upon deposition, can significantly reflow. Reflow of hot sputter deposited materials may result from solid phase and surface diffusion, possibly driven by capillary forces. Reflow may be aided by the thermal energy imparted by the impact of subsequently depositedatoms. Significant reflow preferably encompasses only those forms of bulk redistribution of a metal that occur at elevated temperatures. One benefit of hot sputter deposition of a bulk metal layer is that small voids formed in a bulk metal layer may be filled by reflow of the bulk metal layer into the void.
Many voids cannot, however, be filled simply by reflow of a hot sputtered bulk metal layer. For example, some voids that may form in very small, very high aspect ratio cavities may be too large to refill using only reflow. In addition, cooling of the bulk metal layer before the bulk metal layer has had a chance to reflow into a void as a result of, e.g., subsequent processing, may also prevent voids void-filling capillary action. But since the filling of a void by metal reflow is a relatively slow process, maintaining a bulk metal layer in a xe2x80x9creflowxe2x80x9d condition for a significant period detracts from valuable processing time. Consequently, with the decreasing cavity width and increasing cavity aspect ratio that are ever-present in current integrated circuit fabrication processes, mere reflow of the bulk metal layer into any voids formed in the bulk metal layer is often insufficient to properly fill every void formed during deposition. In other words, bulk metal layer reflow alone often cannot be reliably depended upon as a mechanism for filling voids formed in bulk metal layers arranged in narrow, high aspect ratio cavities.
Therefore, it would be desirable to develop an improved method for fabricating a metallization structure. The desired method should allow for the reliable filling of even extremely small, very high aspect ratio metallization cavities without significant void formation.
The problems discussed above are in large part resolved by the present metallization structure and a method for fabricating such a structure. The present method preferably includes forming a void within a metal layer. The void is encircled with metal and has an internal pressure (i.e., void pressure level) that is preferably approximately equal to the pressure in a deposition chamber in which the metal layer is arranged when the void is formed. Subsequently, the void may be collapsed by increasing a pressure level outside of the void (i.e., collapsing pressure level) significantly above the void pressure level. Increasing a pressure level outside of the void preferably includes a pressure level within the deposition chamber to a collapsing pressure sufficiently above the void pressure to collapse the void. A metallization structure formed by such a process may be substantially void-free, even in narrow, high aspect ratio metallization cavities.
In an embodiment, a portion of the metal layer is arranged within a cavity defined in a dielectric layer. Forming a void within the metal layer then preferably includes reflowing at least the portion of the metal layer arranged within the cavity at a pressure less than a first metal deposition pressure at which the metal layer was deposited. Such reflowing is preferably continued for a reflowing time sufficient to allow the void to form within the portion of the metal layer arranged within the cavity.
The present method preferably facilitates the reliable filling of narrow, high aspect ratio metallization cavities by taking advantage of the tendency of metals deposited in such cavities to form voids. As noted previously, a void often forms in a metal layer that is sputter deposited into a narrow, high aspect ratio cavity. If the void remains in final metallization structure, the void can prevent the structure from performing as desired. The present method, however, preferably collapses the void by increasing the pressure level within the deposition chamber to a collapsing pressure level. The collapsed metal previously surrounding the void thus collects at the bottom of the unfilled portion of the cavity. As such, the void collapse effectively reduces the aspect ratio of the remaining unfilled portion of the cavity. The remaining unfilled portion ofthe cavity can subsequently be more easily filled, which preferably results in a metallization structure having excellent fill characteristics and that is substantially void-free. Further, because the present method preferably collapses any voids formed in the metal layer, concern about whether any voids actually remain (such as is present when relying on reflow alone to fill a cavity) may be greatly reduced.
The present process also preferably obtains such benefits without requiring the incorporation of overly complicated, multiple-chamber bulk metal layer deposition processes. In response to the difficulty that conventional metallization structure formation processes have with small, high aspect ratio cavities, some manufacturers have resorted to using overly complex methods to deposit the bulk metal layer. Some of these methods require multiple chambers for deposition of the bulk metal layer, which may substantially increase the total cost of the metallization process.
But since the present method is preferably configured to collapse the air/gas-filled voids formed in a bulk metal layer deposited in a cavity, the fact that a single chamber metal deposition process may form a void in narrow, high aspect ratio cavities may actually be used to improve the fill characteristics of that process (once the process is modified to accord with the process presented herein). Consequently, the present method may obtain desired process goals without requiring the acquisition and use of overly complicated bulk metal layer deposition processes and equipment, and without suffering the possible extension in overall processing time and cost that may result from forming a bulk metal layer in more than one deposition chamber. In other words, the present method may be used to significantly extend the life of current deposition equipment.
In a preferred embodiment, the bulk metal layer deposition process may be performed in a bulk metal layer deposition chamber. The bulk metal layer deposition process preferably includes cold sputter depositing a cold sputtered portion of the bulk metal layer. A first hot sputtered portion of a bulk metal layer may subsequently be sputter deposited within a cavity defined in a dielectric layer and above the cold sputtered portion of the bulk metal layer. The bulk metal layer deposition chamber preferably has a first metal deposition pressure level during deposition of the first hot sputtered portion of the bulk metal layer. Subsequently, the pressure level within the bulk metal deposition chamber may be reduced to a reflowing pressure level significantly below the first pressure level. Reducing the pressure preferably encompasses the process(es) of initiating a reduction in deposition chamber pressure, achieving the targeted reduced pressure, and maintaining the reduced pressure for a pre-defined amount of time. During pressure reduction, the first hot sputtered portion of the bulk metal layer may be reflowed for a reflowing time sufficient to allow a void to form at least partially within the cavity by reflow of at least a portion of the bulk metal layer arranged within the cavity. The void preferably has an internal pressure of the air or gas therein. The void pressure is preferably approximately equal to the reflowing pressure.
The pressure level within the bulk metal layer deposition chamber and outside of the void may subsequently be elevated to a collapsing pressure level significantly above the reflowing pressure. Concurrent with or subsequent to increasing the pressure within the chamber, a second hot sputtered portion of a bulk metal layer may be hot sputter deposited in the bulk metal layer deposition chamber above the first hot sputtered portion of the bulk metal layer and within the cavity. The bulk metal layer primarily comprises aluminum. Because the present process allows for the formation of a void within a portion of a bulk metal layer arranged in a metallization cavity and for its subsequent collapse, the portion of the bulk metal layer within the metallization cavity is preferably substantially void-free.
Since the collapsing pressure is preferably significantly above the reflowing pressure (and thus, above, the void pressure within the void), void collapse can occur relatively fast. In other words, the collapsed void preferably fills the void much more rapidly than, e.g., reflow of the bulk metal layer could. In addition, collapse of the void preferably results in the remaining portion of the bulk metal layer within the cavity, and specifically within the first hot sputtered portion of the bulk metal layer, being substantially void-free. The present method thus also provides for a more complete, and more rapid, filling of large voids formed in a bulk metal layer than may be possible using only reflow.
In addition, a pre-cleaning process may be performed prior to the bulk metal deposition process. The pre-cleaning process preferably includes sputtering away a portion of the dielectric layer in which the cavity is defined such that the upper portions of the sidewalls of the cavity are tapered. A wetting layer may then be deposited within the cavity and upon the tapered upper portions of the cavity sidewalls prior to cold sputter depositing the cold sputtered portion of the bulk metal layer.
A metallization structure is also presented. The metallization structure preferably includes a bulk metal layer at least partially arranged in a cavity formed in a dielectric layer. The cavity preferably has an aspect ratio of at least 3:1, and more preferably of at least 4:1, and in some instances can properly fill a cavity having an aspect ratio exceeding 5:1. The cavity preferably has a width of at most 0.40 microns, and more preferably of at most 0.25 microns. Preferably, the bulk metal layer substantially fills the cavity and is substantially void-free.